The present invention relates to the technical use of electrical components at an operating temperature below room temperature.
Electrical components such as current limiters, cables or transformers with components made of superconducting materials are suitable for use in power supply networks owing to the reduced resistive losses. However, said components must be cooled to an operating temperature which is considerably below ambient temperature or room temperature. The superconducting components are for this purpose arranged in a coolant container, called a cryostat, and are cooled to the intended operating temperature by coolant, preferably by liquid nitrogen in the case of high-temperature superconducting materials.
Heat is carried into the cryostat via normally conductive input and/or output cables, by means of which the low-temperature components to be cooled are connected to the transmission network, which is at room temperature. These cables are designed such that, during continuous operation, the heat introduced into the cryostat is caused in roughly equal proportions by the resistive heating produced by the rated current flowing in the cables and by heat conduction. A disadvantage in this case is that, even when the electrical current is interrupted, that is to say when the low-temperature apparatus is not being used, heat is still carried through the cables into the cryostat, and corresponding cooling power is required.
Current limiters used in power supply networks and based on superconducters are able to limit potential short-circuit currents to a value close to the rated current which can be carried during continuous operation. EP 0 926 797 A2 discloses an apparatus to limit overcurrents, comprising a parallel circuit formed by an induction coil with a series circuit formed from a high-temperature superconductor and a switch which is closed during normal operation. For its own protection, the superconductor is loaded by the overcurrent for only a short time after a short circuit, for example during the first half cycle, and the load is removed as soon as possible by opening the interrupter. From then on, the short-circuit current is limited by the parallel-connected induction coil. The switch is not closed again until the short circuit has been corrected and the high-temperature superconductor has once again been cooled down to the operating temperature.
It is also known that conventional vacuum switches for the medium-voltage range can be produced cost-effectively. These are normally opened at the zero crossing of a current that is to be interrupted. Their disadvantage is that asymmetrical currents with no zero crossings cannot be switched off safely. U.S. Pat. No. 3,812,314 describes a vacuum switch for underground transformers, which is installed in a plastic bushing for cost and space reasons. The ceramic of the switch housing is in this case completely surrounded by the plastic insulation of the bushing. A flange in the bushing allows it to be connected in a hermetically sealed manner to a wall of a transformer container.
The object is to reduce the heat supplied to a cryostat via an electricity cable leading to a low-temperature component. This object is achieved by a low-temperature apparatus having the features of the present invention.
The essence of the invention is the provision of a vacuum switch to interrupt the electrical cable and the integration of this vacuum switch in a bushing, which leads to the interior of a cryostat, of the electrical cable. The vacuum of the switch thus complements the thermal insulation of the cryostat, which is interrupted in the region of the bushing. Particularly in the switched-off state, that is to say when the switch is open, the introduction of heat into the cryostat is thus considerably reduced.
According to a first embodiment, the vacuum flask of the switch and the bushing insulation are each manufactured from a poorly thermally conductive electrical insulator, and they are preferably actually identical.
According to a second embodiment, the electrical input and/or output cables lead to a component which is located in the cryostat and is based on a high-temperature superconductor. The coolants in this case are preferably liquid nitrogen.
In a further embodiment, the low-temperature component is a superconducting current limiter. This embodiment is based on the knowledge that a short-circuit current is limited symmetrically when a superconductor-based current limiter is used, that is to say no significant direct-current offset components occur. For this reason, a switch which switches safely only at a current zero crossing is sufficient to interrupt the electrical cable to be protected. A combination of a superconducting current limiter and vacuum switch thus likewise has an explicitly inventive character.